//! Represents the path of an `Entity`, describing its place in the world's
//! hierarchy which is constructed using `ChildOf` relationships.

const std = @import("std");
const Allocator = std.mem.Allocator;

const meta = @import("./meta.zig");
const Entity = @import("./entity.zig").Entity;

// TODO: See if we should even be using this?
const native_endian = @import("builtin").cpu.arch.endian();

// TODO: Do something better than just `std.debug.assert`.
// TODO: Offer a way to validate paths, like checking for empty parts.

const Path = @This();

/// Whether the path is specified to be absolute.
/// Note that a relative path can still be interpreted as absolute.
absolute: bool,
/// The string parts that make up the path.
parts: []const EntityPart,
/// The allocator that was used to allocate `parts`.
alloc: ?Allocator = null,
/// Whether the parts outer array itself is owned by this `Path`.
owns_array: bool = false,
/// Whether the parts inner strings are owned by this `Path`.
owns_parts: bool = false,

/// Represents an `Entity` in a `Path`, either by name or its numeric id.
pub const EntityPart = union(enum) {
    id: u32,
    name: [:0]const u8,
};

/// Format used to parse and stringify `Path`s.
///
/// When this type is formatted, you may use any of the `FormatOption`
/// constants in this type by name as `fmt` specifier, such as `"{unix}"`.
/// An empty `fmt` results in the `default` format being used, which can be
/// changed at runtime.
pub const FormatOptions = struct {
    /// The root separator used for an absolute `Path`, if any.
    ///
    /// If set to `null`, absolute paths can't be represented using strings.
    /// In this case, an absolue path and relative path with identical parts
    /// will be indistinguishable.
    root_sep: ?[]const u8,
    /// The separator used between parts that make up a `Path`.
    sep: []const u8,

    /// The format used by Flecs' C API. For example `flecs.core`.
    pub const flecs_c = FormatOptions{ .root_sep = null, .sep = "." };
    /// The format used by Flecs' C++ API. For example `::flecs::core`.
    pub const flecs_cpp = FormatOptions{ .root_sep = "::", .sep = "::" };
    /// Unix-like format. For example `/flecs/core`.
    pub const unix = FormatOptions{ .root_sep = "/", .sep = "/" };

    /// The default format used when none is specified. Can be changed at runtime.
    pub var default = flecs_c;
};

/// Creates an array of `EntityPath`s with known size equal to the number of
/// elements in the specified tuple argument. Each element of the tuple is
/// either converted to a `.name` part, or an `.id` part.
///
/// Keep in mind the mutability and lifetime of the string elements passed
/// to this function, as they aren't cloned and ownership stays the same.
/// In many cases, the lifetime of `Path`s is relatively short. When this
/// is not the case, it's recommended to `.clone()` the path after creation.
pub fn buildParts(parts: anytype) [numElements(parts)]EntityPart {
    if (comptime !meta.isTuple(@TypeOf(parts)))
        @compileError("Expected tuple, got '" ++ @typeName(@TypeOf(parts)) ++ "'");
    var result: [numElements(parts)]EntityPart = undefined;
    inline for (&result, parts) |*res, part| {
        res.* = if (comptime meta.isZigString(@TypeOf(part)))
            .{ .name = part }
        else switch (@typeInfo(@TypeOf(part))) {
            .Int, .ComptimeInt => .{ .id = part },
            else => @compileError("Expected '[:0]const u8' or 'u32', got '" ++ @typeName(@TypeOf(part)) ++ "'"),
        };
    }
    return result;
}

/// Returns the number of elements of the specified tuple type.
fn numElements(parts: anytype) usize {
    return if (comptime meta.isTuple(@TypeOf(parts)))
        @typeInfo(@TypeOf(parts)).Struct.fields.len
    else
        0;
}

/// Creates a new `Path` from the specified parts.
///
/// The resulting path does not own any of the given slices.
pub fn fromParts(absolute: bool, parts: []const EntityPart) Path {
    std.debug.assert(parts.len > 0);
    return .{ .absolute = absolute, .parts = parts };
}

/// Parses a string as a `Path` using the `FormatOptions` specified,
/// or `FormatOptions.default` if the argument is `null`.
///
/// If the string starts with the specified root separator (if any), the
/// resulting path will be absolute. The rest of the string will be split
/// by the specified seperator, becoming its parts.
///
/// This function will allocate duplicate strings taken from the specified
/// source `path`, to ensure they are sentinel-terminated. The resulting
/// `Path` takes ownership of these.
pub fn fromString(path: []const u8, options: ?FormatOptions, alloc: Allocator) !Path {
    if (path.len == 0) return error.MustNotBeEmpty;
    const opt = options orelse FormatOptions.default;

    var remaining = path;
    var absolute = false;

    // If `root_sep` is set and path starts with it, the path is absolute.
    if (opt.root_sep) |p| {
        if (std.mem.startsWith(u8, remaining, p)) {
            remaining = remaining[p.len..];
            absolute = true;
        }
    }

    const parts_len = std.mem.count(u8, remaining, opt.sep) + 1;
    const parts = try alloc.alloc(EntityPart, parts_len);

    var i: usize = 0;
    var it = std.mem.splitSequence(u8, remaining, opt.sep);
    while (it.next()) |str| : (i += 1)
        parts[i] = if (parseNumericId(str)) |id|
            .{ .id = id }
        else
            .{ .name = try alloc.dupeZ(u8, str) };

    return .{
        .absolute = absolute,
        .parts = parts,
        .alloc = alloc,
        .owns_array = true,
        .owns_parts = true,
    };
}

/// Creates a `Path` for the specified `child` entity, optionally in
/// relation to the specified `parent` entity. If `parent` is not `null`,
/// the resulting path is relative. Otherwise it will be absolute.
///
/// This function allocates an array for the parts that make up the entity's
/// path, however each part itself is owned by Flecs and could change or be
/// invalidated ay any time, such as when an entity is renamed or removed.
pub fn fromEntity(comptime ctx: type, parent: ?Entity(ctx), child: Entity(ctx), alloc: Allocator) !Path {
    std.debug.assert(child.raw != 0);
    if (parent) |p| {
        std.debug.assert(p.raw != 0);
        std.debug.assert(p.raw != child.raw);
    }

    // TODO: Use a threadlocal field with reasonable size, then clone the result.
    const starting_capacity: usize = 12;
    var parts = try alloc.alloc(EntityPart, starting_capacity);
    errdefer alloc.free(parts);
    var num_parts: usize = 0;

    // Traverse up the entity hierarchy starting from the specified child
    // entity up until either the specified parent or root of the hierarchy.
    var current = child;
    while (true) {
        std.debug.assert(num_parts == 0 or current.raw != child.raw); // Cycle detected.
        parts[num_parts] = if (current.name()) |name|
            .{ .name = name }
        else
            .{ .id = current.entityId() };
        num_parts += 1;

        // Move to the parent entity, if any.
        current = current.parent() orelse
            // If `parent` wasn't specified, we reached the root. Done.
            // Otherwise, if the parent wasn't found, return an error.
            if (parent == null) break else return error.ParentNotFound;

        // If we reached the specified `parent`, we're done here!
        if (parent != null and current.raw == parent.?.raw) break;
    }

    parts = try alloc.realloc(parts, num_parts);
    std.mem.reverse(EntityPart, parts);

    return .{
        .absolute = parent != null,
        .parts = parts,
        .alloc = alloc,
        .owns_array = true,
    };
}

/// Creates a deep clone of this `Path` using the specified `Allocator`.
pub fn clone(orig: Path, alloc: Allocator) !Path {
    var parts = try alloc.dupe(EntityPart, orig.parts);
    errdefer alloc.free(parts);

    var num_allocated: usize = 0;
    errdefer for (parts[0..num_allocated]) |part|
        if (part == .name) alloc.free(part.name);

    for (parts) |*part| {
        if (part.* == .name)
            part.* = .{ .name = try alloc.dupeZ(u8, part.name) };
        num_allocated += 1;
    }

    return .{
        .absolute = orig.absolute,
        .parts = parts,
        .alloc = alloc,
        .owns_array = true,
        .owns_parts = true,
    };
}

/// Destroys any memory owned by this `Path`, if any.
pub fn deinit(self: Path) void {
    if (self.owns_parts)
        for (self.parts) |part|
            if (part == .name)
                self.alloc.?.free(part.name);
    if (self.owns_array)
        self.alloc.?.free(self.parts);
}

pub fn toString(
    self: Path,
    options: ?FormatOptions,
    alloc: Allocator,
) ![:0]const u8 {
    const opt = options orelse FormatOptions.default;
    const length = self.calculateLength(opt);
    const result = try alloc.allocSentinel(u8, length, 0);
    var stream = std.io.fixedBufferStream(result);
    try write(self, opt, stream.writer());
    return result;
}

pub fn format(
    self: Path,
    comptime fmt: []const u8,
    options: std.fmt.FormatOptions,
    writer: anytype,
) !void {
    _ = options; // TODO: Actually make use of this.
    const opt = fmtToFormatOptions(fmt);
    try self.write(opt, writer);
}

pub fn fmtToFormatOptions(comptime fmt: []const u8) FormatOptions {
    return if (fmt.len == 0)
        FormatOptions.default
    else if (@hasDecl(FormatOptions, fmt) and @TypeOf(@field(FormatOptions, fmt)) == FormatOptions)
        @field(FormatOptions, fmt)
    else
        @compileError("invalid format string '" ++ fmt ++ "'");
}

/// Returns whether the contents of the specified `Path`s are equivalent.
///
/// Path equivalency does not imply these paths are or are not referring to
/// the same `Entity`. For example, an entity that is referred to using its
/// entity id has a different path from the same entity referred to by name.
pub fn equals(first: Path, second: Path) bool {
    if (first.absolute != second.absolute) return false;
    if (first.parts.len != second.parts.len) return false;
    for (first.parts, second.parts) |a, b| switch (a) {
        .id => |a_id| if (b != .id or a_id != b.id) return false,
        .name => |a_name| if (b != .name or !std.mem.eql(u8, a_name, b.name)) return false,
    };
    return true;
}

fn calculateLength(self: Path, opt: FormatOptions) usize {
    // Separators.
    var result = opt.sep.len * (self.parts.len - 1);
    // Root separator.
    if (self.absolute) {
        if (opt.root_sep) |p|
            result += p.len;
    }
    // Parts themselves.
    for (self.parts) |part|
        result += switch (part) {
            .id => |id| numDigits(id),
            .name => |name| name.len,
        };
    return result;
}

fn write(self: Path, opt: FormatOptions, writer: anytype) !void {
    // Write root separator (if applicable).
    if (self.absolute)
        if (opt.root_sep) |sep|
            try writer.writeAll(sep);

    // Write the first part.
    switch (self.parts[0]) {
        .id => |id| try writer.writeInt(u32, id, native_endian),
        .name => |name| try writer.writeAll(name),
    }

    // Write the remaining parts, each preceeded bu separator.
    for (self.parts[1..]) |part| {
        try writer.writeAll(opt.sep);
        switch (part) {
            .id => |id| try writer.writeInt(u32, id, native_endian),
            .name => |name| try writer.writeAll(name),
        }
    }
}

/// Attempts to parse the specified string as a numeric entity id.
fn parseNumericId(str: []const u8) ?u32 {
    if (str.len == 0 or str.len > 10) return null;
    var result: u32 = 0;
    var place: u32 = 1;
    var it = std.mem.reverseIterator(str);
    while (it.next()) |chr| {
        const d = std.fmt.charToDigit(chr, 10) catch return null;
        result = std.math.add(u32, result, d * place) catch return null;
        place *%= 10; // Wrapping to avoid overflow error.
    }
    return result;
}

/// Calculates the numbers of digits of an entity id when formatted as a
/// string. Since entity ids tend to be small this is technically optimized
/// for smaller numbers but performance likely doesn't matter much.
fn numDigits(n: u32) usize {
    // zig fmt: off
    return if (n < 10) 1
      else if (n < 100) 2
      else if (n < 1000) 3
      else if (n < 10000) 4
      else if (n < 100000) 5
      else if (n < 1000000) 6
      else if (n < 10000000) 7
      else if (n < 100000000) 8
      else if (n < 1000000000) 9
      else 10;
    // zig fmt: on
}

test Path {
    const alloc = std.testing.allocator;
    const expect = @import("./test/expect.zig");

    // Paths may be constructed by parsing strings.
    const relative = try Path.fromString("some.relative.path", null, alloc);
    defer relative.deinit();

    // Alternatively they can be made by specifying the individual component
    // parts they're made of, as well as an argument specifying whether it's
    // an absolute path.
    // To do this you can use the `buildParts` helper function, which is less
    // wordy than building `EntityPart` structs manually.
    const absolute1_parts = Path.buildParts(.{ "I'm", "absolute!" });
    const absolute1 = Path.fromParts(true, &absolute1_parts);
    // No need to call `deinit()`. The path does not own the `absolute1_parts`
    // array nor the string parts, which in this case are comptime constants.

    // When handling paths, always be aware of the lifetime of its array and
    // any string parts contained within it. This API allows you to completely
    // avoid allocation if you know what you are doing.
    // In the above example, `absolute1_parts` is an array allocated onto the
    // stack, and as such will only be valid until the end of the scope. This
    // means no allocation is necessary, but it also means `absolute1` is only
    // valid for as long as its parts are.
    // If a path instance is not immediately consumed and you're uncertain
    // about the lifetime of its parts, consider using `.clone(alloc)`.

    // With `options` unspecified, it's not possible to represent an absolute
    // path using a string. Pass your own `FormatOptions` to be able to.
    const absolute2 = try Path.fromString("/home/copygirl", Path.FormatOptions.unix, alloc);
    defer absolute2.deinit();

    // Use `.absolute` to test if the path is absolute.
    // Relative paths can be treated as absolute in the absence of a context.
    // Using absolute paths where a relative one is expected may cause an error.
    try expect.false(relative.absolute);
    try expect.true(absolute1.absolute);
    try expect.true(absolute2.absolute);

    // The internal component parts of the path can be accessed with `.parts`.
    try expect.equal(3, relative.parts.len);
    try expect.equal("some", relative.parts[0].name);
    try expect.equal("relative", relative.parts[1].name);
    try expect.equal("path", relative.parts[2].name);

    // Parts can also be numeric ids, used for entities that don't have a name.
    const numeric1 = try Path.fromString("100.101.bar", null, alloc);
    defer numeric1.deinit();
    try expect.equal(3, numeric1.parts.len);
    try expect.equal(100, numeric1.parts[0].id);
    try expect.equal(101, numeric1.parts[1].id);
    try expect.equal("bar", numeric1.parts[2].name);

    // Numeric ids can also be passed to `buildParts`.
    const numeric2_parts = Path.buildParts(.{ 100, 101, "bar" });
    const numeric2 = Path.fromParts(false, &numeric2_parts);
    try expect.equalDeep(numeric1.parts, numeric2.parts);

    // Paths are formattable. As format specifier you can use options defined
    // on the `FormatOptions` type, or an empty string to use the default.
    try expect.fmt("some.relative.path", "{}", .{relative});
    try expect.fmt("::I'm::absolute!", "{flecs_cpp}", .{absolute1});
    try expect.fmt("/home/copygirl", "{unix}", .{absolute2});

    // They can also be turned directly into strings, which
    // allows you to use entirely custom `FormatOptions`s:
    const absolute1_str = try absolute1.toString(.{ .root_sep = "= ", .sep = " + " }, alloc);
    defer alloc.free(absolute1_str);
    try expect.equal("= I'm + absolute!", absolute1_str);

    // The default `FormatOptions` may be changed.
    Path.FormatOptions.default = Path.FormatOptions.unix;
    // But be sure to unset them after this test so other tests can succeed.
    defer Path.FormatOptions.default = Path.FormatOptions.flecs_c;

    // This affects functions that use them to parse or format strings.
    try expect.fmt("some/relative/path", "{}", .{relative});
    const another_relative = try Path.fromString("mom/sister/child", null, alloc);
    defer another_relative.deinit();
    try expect.fmt("mom/sister/child", "{}", .{another_relative});

    // A deep clone of a path can be allocated using `.clone()`.
    // This clone owns the outer array and its inner strings.
    const twin = try another_relative.clone(alloc);
    defer twin.deinit();
    try expect.fmt("mom/sister/child", "{}", .{twin});
}